The present invention relates to a device for measuring a physical quantity, this device comprising means for producing an alternating electric signal having a frequency representing the quantity.
Is is advantageous to measure some physical quantities, in particular force and temperature, in this way. When for instance a force is measured, this force may be applied directly to the device or may for example be derived from a pressure exerted on the device or from an acceleration which the device undergoes. In the latter case, the device performs as an accelerometer.
A force-measuring device of this type is for example described in detail in European Pat. No. EP-B-0 098 796 and the corresponding U.S. Pat. No. US-4 498 344. The device described therein includes a resonator, formed as a dual tuning fork fixed on a deformable support to which the force to be measured is applied whereby this force modifies the oscillation frequency of the resonator, and a conventional driving circuit adapted to vibrate the resonator at this frequency. The resonator and the driving circuit thus form an oscillator, the circuit being so arranged that it supplies an output signal made up by a sequence of pulses, the frequency of repetition of these pulses, i.e. the frequency of the oscillator, thus representing the force applied to the device. This frequency is then measured by a conventional counter and next calibrated, by means of a digital calculator circuit, into units of force or, possibly, into units of pressure or acceleration. Finally, the data supplied by the calculator circuit, representing the constraint applied to the resonator, is indicated by a digital display.
In this device, the frequency of the signal supplied by the oscillator, which is an analog quantity, is nevertheless measured and then processed by logic circuits in such a manner that the final result appears in digital form. Given that the oscillator signal has previously been converted to a sequence of pulses, this is the most precise, reproducible and reliable way of measuring the force applied to the device.
For some applications, it is however preferable to show the result in analog form, i.e. the result should be given by the amplitude of a signal that varies continuously, in dependence on the force to be measured and hence of the frequency of the signal supplied by the oscillator. This can be done by means of known analog circuits providing a continuous output signal whose amplitude is dependent on the frequency of the input signal. Such circuits however have the drawback of becoming somewhat imprecise and unstable with time, because their characteristics depend on components subject to ageing. It is also possible to employ logic circuits, also of known type, that directly convert a digital signal into an analog signal. While these circuits do preserve the precision and the stability of the digital measurement, they suffer from the disadvantage of being complex and expensive.